Transmission line circuit



Aug. 12, 1947- N. E. LINDENBLAD TRANSMISSION LINE CIRCUIT Filed Sept. 4, 1943 ATTORNEY Patented Aug. 12, 1947 TRANSMISSION LINE CIRCUIT Nils E. Lindenblad, Rocky Point, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application September 4, 1943, Serial No. 501,273

, 7 Claims. 1

The present invention relates to high frequency filter systems for connecting together one or more transmitters and receivers to a common antenna system and, more particularly, to such systems for use in pulse transmitting and receiving systems used in radio detecting and ranging.

An object of the present invention is the provision of a filter device for enabling the selective coupling of atransmitter and receiver to a common antenna system without requirin the use of mechanical switches.

Another object of the present invention is the provision of a filter network, the operating characteristics of which are controlled by the power applied thereto.

A further object of the present invention is the provision of a filter device for pulse transmitting and receiving systems.

Still a further object is the provision of an antenna and filter device for pulse transmitting and receiving equipment which utilizes no moving parts or mechanical switches for diverting the outgoing and incoming pulses to the appropriate transducer equipment.

Still a further object is the provision of a filter device, as aforesaid, particularly designed for use in systems utilizing crystal receivers.

Still a further object is the provision of a filter device, as aforesaid, in which the power consumption of the device is small.

Still another object is the provision of a filter circuit which utilizes low power trapping sparks for performing the switching operation.

The foregoing objects, and others, which may appear from the following detailed description, are attained: in: accordance with the principles of the present invention by providing a filter device between anantenna, a receiver and a transmitter, the filter device connecting. the transmitter, receiver and antenna together in a bridge fashion. The: bridge of the. filter device utilizes a plurality of resonant structures having electrical discon= tinuitie's therein and which are so placed. and so connected that when signalling potentials. are applied thereto from the transmitterthe electrical discontinuities are caused tob'reak'down and become conducting. The. filter device thereby causes the energy from the transmitter to be conducte'dtothe-antenna but prevents'its application to the receiver. When the transmitter ceases op-' eration, the electrical discontinuities arerestored and the signals picked up by the antenna are ap plied to. the, receiver. The electrical disc'ontinuie tiesare so placed that no" received energy is. per:- mitted to flow to the transmitter circuit;

The present invention will be more completely understood by reference to the following detailed description, which is accompanied by a drawing in which transmission line TL is adapted to be connected from a suitable antenna (not shown) to the filter device of the present invention; transmission line TLT is adapted to be connected from a transmitter organization (not shown) to the filter device, while transmission line TLR is adapted to connect the receiver, which is likewise not shown, to the filter device. Transmission line TLT includes a hollow tubular. outer shell 3 concentrically arranged about an inner conductor 4 and transmission line TL includes an outer shell 'I and a concentrically arranged inner conductor 2. Between transmission lines TL and TLT are connected two parallel branch concentric transmission lines 1 and I0. Transmission line I includes an outer shell 8 and a coaxially arranged inner conductor 9 and transmission line ID has an outer shell II and an inner conductor I2. The ratio of the diameters of shells I and 3 to the inner conductors 2 and 4 associated there with may be so chosen as to provide a characteristic impedance of the order of '70 ohms. Each of the branch transmission lines 1 and I0 may have the ratios of the inner and outer conductors so chosen as to provide a characteristic imped ance of the order of 100 ohms. However, the present invention is not limited to the use of these values of impedance which are given only as illustration. TL and TLR may, for instance, be ohms, in which case they match the branch lines.

Now, if it is assumed that the transmitter is in operation and the power pulse from the transmitter traveling along transmission line TLT arrives at the junction of transmission lines 1 and ill with transmission line TLT at point A, the power pulse may then proceed along the two branch lines 7 and H) to point F where they join with transmission line TL. Transmission line 1 at point B has connected thereto a low power spark trap l 5 while transmission line ID has connected thereto at point C a similar spark trap I6. Spark trap I5 includes an outer tubular shell I connected at one end to a shell 8 of transmission line I and an inner conductor l8 connected at one end to the inner conductor 9 of transmission line I at point B. The other end of shell I! is closed by an end plate and the adjacent end of inner conductor [8 has interposed between it and the end plate a spark gap I9. Similarly, the spark trap l6 includes an outer shell 20 connected to-outer shell ll of transmission lineliland: an

3 inner conductor 2| connected at point C of conductor I2 of transmission line 10. The end ofconductor 2|, remote from point C, is connected to the closed end of shell 20 by a spark gap 22.

When the previously mentioned pulse from the transmitter arrives at points B and C on the transmission lines "I and IE3, spark gaps is and '22 break down thus directly connecting conduc- Y tors i8 and 2| to the outer shells l1 and 29. Due to the quarter wave-length of these trap elemerits, points B and C assume a'high impedance for the energy of the pulse and the power pulse thus proceeds along the branches as though there were no traps at all.

The transmission line TLR is connected in a push-pull relationship to the two inner conductors 9 and I! to transmission from the receiver 1 lines 1 and H) at points D and E, m thane outer shell 5 of transmission line TLR is directly connected at its upper end to conductor l2 of transmission line TL, while the inner conductor of transmission line 6 of conductor TLR, is directly connected to conductor 9 of transmission line i. In'order to electrically free the end of the outer shell of transmission line TLR from ground, a quarter'wave end portion 3'6 is 'sur-.

portion 30.0f theouter shell 5 of transmission line TLR'. This cylindrical conductingportion 33 is surrounded by an outer shell 34 towhich 7 it is connected'at a point one quarter of the operating wavelength distant from point D.

Now, since the receiver line TLR, is connected across the two branch transmission lines '1. and H at points. equally distant over each parallel branch from junction A to the line T-LT, there will be no potential difierence across the receiver line TLR developed by the transmitted pulse. None of the pulse energy'will therefore be dissipated inthe receiver circuits, Conductor portions 3i) and 33 act as quarter wave resonant shunts across the transmission lines land l0 and dissipate none of the energy of the pulse. It should be noted that the distance between junctions A and F is chosen to be equal to one wavelength. The connection between junctions A and F consists of two one hundred ohm, lines in parallel. Such a combination does not directly match a '36 ohm line, but this does not matter since the length of the branches is equal to a multiple of a half wave. No matter what the impedance is at one junction, the same impedancewill occur at the other junction, even if there are standing waves on the transmission presents a low impedance at its mouth as if it were grounded since thedistance from point E at the open end of the sleeve extension 30 is a half wavelength from point C where thelow impedance is presented. Since point E is at low impedance the transmission line 10 at point F presents a high impedance to energy arriving along transmission line TL and no'received power will flow over transmission line 1E. Instead, the incoming power will flow along transmission line i to point D where the inner conductor 9 of transmission line a is connected to the inner conductor 6 of transmission line TLR. The received energy will therefore be applied from the antenna to the receiver. Since spark trap 55 is'located a distance of a quarter wavelength away from point D and presents a low impedance at point B, transmission line 1 at point D will present a high impedance to the flow of energy further along transmission'iline '1. .Thusjthe passing of any energy from the antenna to the transmitter is prevented.

The trap circuits 15' and It become the most efiective when the line elements have the highest characteristic impedance. This is due to the fact that the spark impedance is not zero and thus the highest impedance is obtained a quarter wave distant from the spark aps when the transforming link, that is, the distance along the con: ductor between gap 19, for example, and pointB has a characteristic impedance well above that of gap I9 itself because the characteristic impedance of a quarter wave transforming link forms the geometric mean of the impedances at its terminals. 7 r

It will be notedthat the first length of conductor 6 within the shell extension 30 is given such diameter with relation to the shell 30 as to present a characteristic impedance of ohms. The distance from point B through point D to a point on conductor" 6 opposite 32 on the outer shell 5 being a full half Wave,- there will be no interruption in line matchingfbetween transmission line TL- and transmission line; TLR. Between point 32 on the transmission line TLR. and the receiver, the transmission line may have an impedance of 70 ohms, or if another valueis desired, an impedance matching circuit may be interposed.

While I have illustrated a particular e mbodi ment of the present invention, it should be'clearly understood that it is not limited thereto since many modifications maybe made in the several elements employed and in their arrangement and it is therefore contemplated by the appended claims to cover any such modifications asifall within the spirit and scope of the invention. 7 Having now described the invention, what is claimed and desiredto be secured by Letters Patent is the following: 1,

1. A switching circuit adapted to be connected between a first and a second pair of transmission lines and including a pair of parallel transmission lines, each of said transmission lines. in-

cluding anouter shell conductor and an inner conductor, the conductors of said pair of transmission lines being connected in aparallel relationship to conductors of' said first and second lines, the length of each of saidnpair of lines being one wavelength, means for connecting the conductors of a third transmission line in a; pushpull relationship to inner conductors of said pair of lines at adistanceequal to a quarter-of the operating wavelength from the connection to the first transmission line,a quarterwave trap spark circuit connected across one of said pair of lines at a distance equal to one quarter of the operat ing wavelength from th other end and a quarter wave trap spark circuit connected across the other of said pair of lines at a distance equal to one half of V the operating Iwavelength fromsaid other end, f' ,i I 1:. i, I Tiff:

quarter wave-P t 2. A switching circuit adapted to be connected between a first and a second pair of transmission lines and including a pair of parallel transmission lines, each of said transmission lines including an outer shell conductor and an inner conductor, the conductors of said pair of transmission lines being connected in a parallel relationship to conductors of the first and second lines, the length of each of said pair of lines being an even multiple of a half of the operating wavelength, means for connecting the conductors of a third transmission line in a push-pull relationship to the inner conductors of said pair of lines at a distance equa1 to a quarter of th operating wavelength from the connection to the first transmission line, a quarter wave trap spark circuit connected across one of said pair of lines at a distance equal to one quarter of the operating wavelength from the other end and a quarter wave trap spark circuit connected across the other of said pair of lines at a distance equal to one half of the operating wavelength from said other end.

3. A switching circuit adapted to be connected between an antenna transmission line and a transmitter transmission line, said circuit including a pair of line sections connected in parallel relationship at adjacent ends of said transmission lines, means for so coupling a receiver transmission line in push-pull relationship to a conductor of each of said pair of line sections that normally different impedances are presented to said line sections by each of the conductors of said receiver transmission lin whereby said receiver is coupled to said line sections, and means responsive to energy from said transmitter to cause the conductors of said receiver transmission line to present the same impedance to energy on said pair of line sections whereby said receiver transmission line is not coupled to said antenna transmission line.

4. A switching circuit adapted to be connected between an antenna transmission line and a transmitter transmission line, said circuit including a pair of coaxial transmission line sections each having an outer shell and an inner conductor and connected oin parallel relationship at adjacent ends of said transmission lines, the length of each of said pair of line sections being an even multiple of one-half wavelength, means for coupling a receiver transmission line in a push-pull relationship to the inner conductors of said pair of line sections, and means for causing the conductors of said receiver transmission line to present different impedances to energy on said pair of line sections whereby said receiver line is coupled to said antenna transmission line, and means responsive to energy from said transmitter for so unbalancing said line sections that the same impedance is presented by each of said conductors of said receiver transmission line to said line sections, whereby said receiver is uncoupled from said pair of line sections.

5. A switching circuit adapted to be connected between an antenna transmission line and a transmitter transmission line, said circuit including a pair of coaxial transmission line sections, each having an outer shell and an inner conductor and connected in parallel relationship at adjacent ends of said transmission lines, the length's of each of said pair of line sections being one wavelength, means for coupling a receiver trans"- mission line in a push-pull relationship to said line sections, said receiver transmission line presenting the same high impedance to energy from said transmitter, and means for effectively connecting the inner conductor of one of said line sections to the outen -shell at the point of connection of one of the conductors of said receiver transmission line, whereby the conductors of said receiver transmission line present different impedances to energy on said pair of line sections, whereby said receiver transmission line is coupled to said antenna transmission line.

6. A switching circuit adapted to be connected between a first and second transmission line, said switching circuit including a pair of parallel transmission line sections, each of said transmission line sections including an outer shell conductor and an inner conductor, the conductors of said pair of transmission line sections being connected in a parallel relationship to conductors of said first and second lines, the length of each of said pair of line sections being one wavelength, means for connecting the conductors of a third transmission line in a push-pull relationship to inner conductors of said pair of line sections at a point intermediate their ends, a pair of quarter-wave trap circuits each having a series spark gap therein, one of said trap circuits being connected across each of said pair of line sections, the point of connection of said trap circuits being so chosen that in the absence of energy above a predetermined value in said line sections, the conductors of said third transmission line present unequal impedances to said pair of transmission line sections whereby said third transmission line is coupled to said first transmission line, the point of connection of said trap circuits being further so chosen that in the presence of energy on said transmission lines above a predetermined value, said trap circuits cause the conductors of said third transmission line to present the same impedances to energy on said pair of line sections, whereby said third transmission line is uncoupled from said first transmission line.

7. A switching circuit adapted to be connected between an antenna transmission line and a transmitter transmission line, said circuit including a pair of line sections connected in parallel relationship to adjacent ends of said other lines, means for coupling a receiver transmission line in a push-pull relationship to a conductor of each of said pair of line sections, and means responsive to energy from said transmitter for causing the conductors of said receiver transmission line to present the same high impedance to energy from said transmitter, said energy responsive means including series spark gaps in quarter wave line sections so connected to said pair of line sections that when said gaps are non-conducting the conductors of said receiver line show different impedances to energy on said pair of lines whereby said receiver line is coupled to said antenna transmission line, said energy responsive means being further so connected to to said pair of line sections that when said gaps are conducting the conductors of said receiver line present the same impedance to energy on said pair of lines whereby said receiver line is uncoupled from said antenna transmission line.

NILS E. LINDENBLAD. 

